This invention relates to a bubble ink jet printing system and, more particularly, to a printhead which is constructed so as to effectively control heat generated during the printing operation.
Bubble jet printing is a drop-on-demand type of ink jet printing which uses thermal energy to produce a vapor bubble in an ink-filled channel that expels a droplet. A thermal energy generator (printhead), is located in the channels near the nozzle a predetermined distance therefrom. A plurality of resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink is ejected from a nozzle and is contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separating of the bulging ink as a droplet. The acceleration of the ink out of the nozzle in which the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a recording medium, such as paper.
A problem with prior art printhead operation is the increase in temperature experienced by a printhead during an operational mode. With continued operation, the printhead begins to heat up, and the diameter of the ink droplet begins to increase resulting in excessive drop overlap on the recording media thereby degrading image quality. As the printhead experiences a further heat buildup, the ink temperature may rise to a point where air ingestion at the nozzle halts drop formation completely. It has been found that, at about 65.degree. for a typical ink, printhead operation becomes unreliable. There is also a lower temperature limit for reliable operation which varies for different inks and device geometries. This limit might, for example, be about 20.degree. C. for an ink and device designed to function reliably up to, for example, 60.degree. C. At the same time, it is desirable to offer an extended range of ambient operating temperatures, such as 5.degree. C. to 35.degree. C., so that it will be necessary to provide for warming up the printhead. It is also desirable to minimize the time required to warm up the printhead, so that first copy (print) out time is acceptable. The printhead characteristics and machine environment requirements have the following impact on the thermal design of the system. The generation of heat during operation (which becomes a greater problem as print speed, duration, and density increase) makes it necessary that the printhead be connected to a heat sink, which is efficient in transferring heat away from the printhead. The efficiency of the heat transfer away from the printhead will be aided increasingly, the cooler the heat sink is relative to the printhead. Because of the range of ambient temperatures to be encountered (assumed to be 5.degree. C. to 35.degree. C., but not limited to that range), because of the temperature uniformity requirement, and because it is less complicated (cheaper) to control temperature by heating than by cooling, it is advantageous to set the nominal printhead operating temperature at or near the maximum ambient temperature encountered. Because of the desired minimal first copy (print) out time, as well as the desired efficiency of the heat sink, it is also advantageous to situate the temperature sensor and heater as close as possible (thermally) to the printhead, and as far as possible (thermally) from the heat sink.
Various techniques are known in the prior art to control heat buildup and maintain the printhead within a reasonable printing temperature range. U.S. Pat. No. 4,496,824 to Kawai et al. discloses a thermal printer which includes circuitry to measure printhead temperature, compare the temperature to values representing a desired temperature range and reduce the printhead temperature by activation of a cooling mechanism.
U.S. Pat. No. 4,571,598 discloses a thermal printhead in which a heat sink and ceramic substrate are connected to heating elements formed on the substrate surface.
More exact temperature regulation is obtained, however, by using a combination of a temperature sensor and a heater in a feedback loop tied into the printhead power source. For example, U.S. Pat. No. 4,250,512 to Kattner et al. discloses a heating device for a mosaic recorder comprised of both a heater and a temperature sensor disposed in the immediate vicinity of ink ducts in a recording head. The heater and sensor function to monitor and regulate the temperature of a recording head during operation. Column 3, lines 7-24 describes how a temperature sensor, a thermistor, a heating element, and a resistor operate in unison to maintain the recording head at an optimum operational temperature to maximize printing efficiency. U.S. Pat. No. 4,125,845 to Stevenson, Jr. discloses an ink jet printhead temperature control circuit which uses a heater and a temperature sensing device to maintain a recording head temperature above the preset temperature level. An output from the temperature sensing device drives an electrical heater which regulates the recording head temperature. The temperature sensing device is a resistive element attached to the bottom side of the printhead by thick film techniques. U.S. Pat. No. 4,704,620 to Ichihashi et al. discloses a temperature control system for an ink jet printer wherein the temperature of an ink jet printhead is controlled by a heater and a temperature sensor which collectively regulate heat transfer to maintain an ink jet printhead within an optimum stable discharge temperature range. The temperature control circuit as shown in FIG. 7 of the patent, utilizes an output from a comparator circuit and control signals from a signal processing circuit to regulate printhead temperature based on the output from the temperature sensor. U.S. Pat. No. 4,791,435 to Smith et al. discloses a thermal ink jet printhead temperature control system which regulates the temperature of a printhead via a temperature sensing device and a heating component. The temperature sensing device, comprised of either a collection of transducers or a single thermistor closely estimates the temperature of the ink jet printhead and compensates for an unacceptable low printhead temperature by either cooling or heating the printhead as needed. U.S. Pat. No. 4,686,544 to Ikeda et al. discloses a temperature control system for "drop-on-demand" ink jet printers wherein a heat generating electrode, positioned between layers of insulating and resistive material of a printhead substrate, controls the temperature of the printhead during operation, Column 4, lines 7-25, describes how an electrothermal transducer delivers the heat required to maintain the ink jet printhead at an optimum temperature level to maximize efficiency printing efficiently. U.S. Pat. No. 4,636,812 to Bakewell, while disclosing a thermal printhead, also teaches using a heater and temperature sensor supported within a laminated layer near the marking resistors.
The above references disclosing the heater and temperature sensor combination may not be suitable for some printing systems depending on factors such as printhead geometry, print speed, ambient operating temperature range, etc.. Further, more exact regulation may be required which is not achievable with the prior art structures. The ideal solution is to form the heater and sensor in close proximity to the printhead in an inexpensive and simple manner. The present invention is directed towards a printhead heat control structure wherein the heater and temperature sensor are formed on the same substrate as that on which the printhead is mounted using thick film screen printing and firing techniques. In a preferred embodiment a metal substrate is used with dielectric and conductive layers formed on a recess in its surface by a selective printing process. More particularly, the invention relates to an improved temperature control system for an ink jet printer which includes an ink jet printhead bonded to an underlying heat sink substrate, the control system including a sensing means for sensing the temperature of said printhead, heater means thermally coupled to said printhead, and a heat sink member in thermal communication with said printhead; control means responsive to outputs from said temperature sensing means and adapted to provide or remove power from said heater means, the improvement wherein said temperature sensing means and heater means are resistive layers separated from said heat sink substrate and said printhead by dielectric layers.